1. Field of the Invention
The present invention relates to gyroscopes, and in particular to mesoscale disc resonator gyroscopes or isolated planar mesogyroscopes and their manufacture.
2. Description of the Related Art
Mechanical gyroscopes are used to determine direction of a moving platform based upon the sensed inertial reaction of an internally moving proof mass. A typical electromechanical gyroscope comprises a suspended proof mass, gyroscope case, pickoffs, forcers and readout electronics. The inertial proof mass is internally suspended from the gyroscope case that is rigidly mounted to the platform and communicates the inertial motion of the platform while otherwise isolating the proof mass from external disturbances. The pickoffs that sense the internal motion of the proof mass, the forcers that maintain or adjust this motion and the readout electronics that must be in close proximity to the proof mass are internally mounted to the case, which also provides the electrical feedthrough connections to the platform electronics and power supply. The case also provides a standard mechanical interface to attach and align the gyroscope with the vehicle platform. In various forms, gyroscopes are often employed as a critical sensor for vehicles such as aircraft and spacecraft. They are generally useful for navigation or whenever it is necessary to autonomously determine the orientation of a free object.
Older conventional mechanical gyroscopes were very heavy mechanisms by current standards, employing relatively large spinning masses. A number of recent technologies have brought new forms of gyroscopes, including optical gyroscopes, such as laser gyroscopes and fiberoptic gyroscopes, as well as mechanical vibratory gyroscopes.
Spacecraft generally depend on inertial rate sensing equipment to supplement attitude control. Currently, this is often performed with expensive conventional spinning mass gyros (e.g., a Kearfott inertial reference unit) or conventionally-machined vibratory gyroscopes (e.g., a Northrup Grumman hemispherical resonator gyroscope inertial reference unit). However, both of these are very expensive, large and heavy.
In addition, although some prior smaller, micromachined symmetric vibratory gyroscopes have been produced, their vibratory momentum is transferred through the case directly to the vehicle platform, so they are not isolated. This transfer or coupling admits external disturbances and energy loss indistinguishable from inertial rate input and hence leads to sensing errors and drift. One example of such a vibratory gyroscope may be found in U.S. Pat. No. 5,894,090, which is incorporated by reference herein, and which describes a symmetric cloverleaf vibratory gyroscope design. Other planar tuning fork gyroscopes may achieve a degree of isolation of the vibration from the baseplate; however, these gyroscopes lack the vibrational symmetry desirable for tuned operation.
In addition, shell mode gyroscopes, such as the hemispherical resonator gyroscope and the vibrating thin ring gyroscope, are known to have some desirable isolation and vibrational symmetry attributes. However, these designs are not suitable for or have significant limitations with thin planar microfabrication. The hemispherical resonator employs the extensive cylindrical sides of the hemisphere for sensitive electrostatic sensors and effective actuators. However, its high aspect ratio and three-dimensional (3D) curved geometry is unsuitable for inexpensive thin planar microfabrication. The thin ring gyroscope (e.g., U.S. Pat. No. 6,282,958, which is incorporated by reference herein), while suitable for thin planar microfabrication, lacks electrostatic sensors and actuators that take advantage of the extensive planar area of the device. Furthermore, the symmetry of shell-mode gyroscopes is inherently limited by the average mechanical precision of only the two machining cuts used to define the inner and outer surface. Moreover, the electrical baseplate or case for this gyroscope is not of the same material as the resonator proof mass so that the alignment of the pickoffs and forcers relative to the resonator proof mass change with temperature, resulting in gyroscope drift. This drift issue is further compounded when the electrical base or case of the gyroscope chip is mounted flat to a platform of dissimilar material, as typical with electronic chip components.
Vibration isolation using a low-frequency seismic support of the case or of the resonator, internal to the case is also known (e.g., U.S. Pat. No. 6,009,751, which is incorporated by reference herein). However, such increased isolation comes at the expense of proportionately heavier seismic mass and/or lower support frequency. Both effects are undesirable for compact tactical inertial measurement unit (IMU) applications because of proof mass misalignment under acceleration conditions.
Furthermore, the scale of previous silicon microgyroscopes (e.g., U.S. Pat. No. 5,894,090, which is incorporated by reference herein) cannot be optimized for navigation or pointing performance resulting in higher noise and drift than desired. This problem stems from dependence on out-of-plane bending of thin epitaxially grown silicon flexures to define critical vibration frequencies that are limited to 0.1% thickness accuracy. Consequently, device sizes are limited to a few millimeters. Such designs exhibit high drift due to vibrational asymmetry or unbalance and high rate noise due to lower mass which increases thermal mechanical noise and lower capacitance sensor area which increases rate errors due to sensor electronics noise.
High value commercial or military applications require much higher inertial quality. However, millimeter (mm) scale micromachined devices are inherently less precise and noisier than centimeter (cm) scale devices, for the same micromachining error. Scaling up of non-isolated silicon microgyroscopes is also problematic because external energy losses will increase with no improvement in resonator Q and no reduction in case-sensitive drift. An isolated cm scale resonator with many orders of magnitude improvement in 3D manufacturing precision is required for very low drift and noise pointing or navigation performance.
For high mechanical quality (Q>1,000,000) needed for low drift sensors, thermoelastic damping must be very low. To minimize mechanical vibrational energy loss through thermal energy dissipation, the gyroscope's elements must vibrate either adiabatically or isothermally. Silicon is highly thermally conductive, and therefore thin elements, e.g., 2.5 microns wide, for isothermal vibration have been commonly used in MEMS designs, i.e., thermal relaxation time is much shorter than the vibration period. More precisely micromachined thick silicon beams would be impractically thick for very long thermal relaxation times and effective adiabatic operation.
Fused quartz, PYREX, or silicon-germanium (SiGe) alloy, on the other hand, is much less thermally conductive, so that practically thick beams can be used with adiabatic vibration, i.e., thermal relaxation time is very long relative to the vibration period. At mesoscale, the required element thickness, ˜100 um, is practical and, for the same fixed etching error, e.g., 0.1 micron, yields much more precisely symmetric micromachined devices than at microscale, e.g., <10 um, as well as much increased mass and reduced thermal noise and much increased area and hence reduced capacitive sensor noise. Low thermal conductivity materials with low thermal expansion coefficient coupling thermal to mechanical energy, e.g., fused quartz, have been discovered to be remarkably ideal for adiabatic vibration with low thermoelastic damping and feasible to micromachine for a planar mesoscale resonator. The higher volume to surface ratio inherent with mesoscale vs. microscale devices results in the reduced effect of surface related damping on overall mechanical quality, such as losses at or within any conductive layer, or losses due to surface roughness. Conventionally machined navigation grade resonators, such as chrome-plated fused quartz hemispherical or shell gyros, have the optimum noise and drift performance at large scale, e.g., 30 mm and 3D manufacturing precision; however, such gyros are expensive and difficult to manufacture.
The low thermal conductivity desired for adiabatic operation comes at some cost, as the materials that have low thermal conductivity also tend to be electrically insulating (with the exception of SiGe, which can be made sufficiently electrically conductive by bulk doping). This feature must be dealt with as electrical conductivity is necessary for the electrostatic driving and sensing of the resonator. In particular, fused silica, a material that has the best thermoelastic properties at the mesoscale of those commonly available, is also a very good insulator. To overcome this, a very thin conductive film is deposited onto the resonator and electrode surfaces. This film provides adequate conductivity (low enough resistance so that the electronics do not pick up additional noise and parasitic signals) while not affecting the mechanical Q. Preferably, the film is very thin and uniform.
There is a need in the art for a micromachined, Coriolis-sensing, mesogyroscope with thick mesoscale, adiabatically vibrating elements and an electrically conductive resonator for electrostatic sensing, actuation and trimming. Specifically, there is a need for a mesogyroscope that has lower cost and higher precision than one-at-a-time, conventional, 3D machined, mesoscale, Coriolis-sensing gyroscopes, and that has higher mechanical precision and performance than other micromachined gyroscopes with thin, microscale, isothermally vibrating elements or micromachined, mesoscale, silicon gyroscopes. There is also a need for a mesogyroscope that also has higher performance due to its electrically conductive resonator permitting highly sensitive capacitive or tunneling sensing and capacitive actuation as compared to micromachined gyroscopes with piezoelectric materials or sensing and actuation elements attached. As detailed below, the present invention satisfies all these and other needs.